Some sell those aluminum sealed boxes for the computer under the seat and they are expensive, a friend of mine used a good quality tupperware box with lid and carefully cut the slots and holes needed for the cables and sealed with silicon, it seems to work.
I don't have the problem with water anymore since I have the 911 style hardtop conversion and that eliminates water issues (still need the drain pans to be in good shape with this hardtop)
I would just like to add: There are also drains in the cowling that have to be kept free flowing.
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i wound up filling the hole with goop, letting it dry fully, and covering with the flashing tape
it turns out the thing under the drivers seat is the rear control module. i ordered a new one and the waterproof box, we'll see